1. Field of the Invention
This invention belongs to the luminous flux measurement field, and especially relates to the equipment and method for LED's total luminous flux measurement with a narrow beam standard light source.
2. Description of the Related Art
After having caused the Micro-electronics Revolution, the Semiconductor Technology is causing another industry revolution—the Illumination Revolution. The development, production and application of Light Emitting Diodes (LEDs) has increased tremendously in the last ten years. From their humble beginnings as panel indicator lights, they are now available in many shapes, sizes, light output levels and colors, making them suitable candidates for use in traffic signaling systems, automobile lights and in general lighting applications. The development of the new high power LEDs with a significant increase in efficiency over the earlier versions has resulted in an advantage of close to an order of magnitude of the LED over color filtered incandescent lamps. In china's 11th Five-Year Plan, the development of high-luminous-efficiency LED has been preferentially supported. Supports are given not only to the research for increasing LED's luminous efficiency, but also to LED's parameters measurement technologies such as LED's luminous efficiency measurement.
In fact, difficulties for luminous efficiency measurement of light sources lie in the total luminous flux measurement. Since the geometry structure of LED is different from traditional light sources, there is not yet an international standard method for LED's luminous flux measurement, although the International committee on Illumination (CIE) and research institutions of America and Canada have suggested some measurement method for LED's total luminous flux.
Some Challenging Problems in Total Flux Measurement for LED:
1) The Mismatch Between Sensitivity of Detector R(λ) and Spectral Luminous Efficiency Function V(λ).
We use incandescent lamp for calibrating photometer currently and evaluate the mismatch error (refer as SCF) in average value over the whole visible range, this evaluation defines the average error in percentage for whole visible range while the percentage error in blue wavelength is possibly quite bigger than SCF since the absolute value of V(λ) is very small in the blue wavelength range, similar situation occurs in red wavelength range. Photometer with good SCF calibrated in the methods is good for measuring light sources with continuous spectrum. However, LED is available in wide variety of peak wavelengths covering the visible and adjacent wavelength range, and LED's narrow bandwidth is typically 20 nm to 40 nm. Therefore, it will cause a significant error in total flux measurement for blue/red LED by a photometer even with a good SCF correction.
2) Great Absorption of Objects Inside the Sphere:
For an ideal sphere calculation, it is assumed that there are no objects in the sphere. However, when we use a sphere, we need to put objects (the light sources, baffles and the holders for the light sources) into it, these objects will inevitably cause disturbance of light distribution within the sphere and error in measuring of LED's total luminous flux.
3) Self-Heating Problems
LED is sensitive to the environment temperatures, so the temperature of LED must stay stable during the measurement. When we place the LED inside the integrating sphere, it can't dissipate its heat easily, which will cause the drop of output luminous flux, and bring measurement errors.
Because of these problems, there are always disagreements in LED's luminous flux measurement, which affects LED's performance judgment, and is not benefit for the development of the LED's industry.